The present invention relates to a DC-to-AC power inverter. In particular, the present invention relates to an inverter circuit having an output waveform which approximates a sinusoid.
Power inverters are often used to control variable-speed AC motors, or to power AC loads from a DC power source. The conventional single-phase inverter circuit consists of a four electronic switches connected in an H-bridge configuration, and fed by a common DC voltage source. Each electronic switch typically consists of a thyristor or IGBT and a free-wheeling diode connected across the thyristor (IGBT). When the conduction interval of the respective switches is properly synchronized, the switches generate a square wave voltage signal between their respective outputs. The square wave output is then passes through a low-pass or integrating filter to thereby produce a sinusoidal output voltage signal.
Pulse-width modulation techniques are typically used to control the magnitude of the output voltage signal applied to the load. PWM-based inverters are advantageous, since the electronic switches can be commutated using a relatively simple control unit. However, PWM-based inverters typically generate high amplitude harmonics, thereby increasing the size of the low-pass filter required. Although harmonic distortion can be reduced by increasing the modulation frequency, this solution compromises efficiency since switching losses are proportional to the number of switch commutations per cycle. Also, since switching losses are proportional to the amplitude of the output voltage, it has been difficult to efficiently generate large output voltages using a PWM-based inverter. Accordingly, attempts have been made to develop an improved mechanism for converting DC power into AC power.
For instance, Baker (U.S. Pat. No. 4,117,364) teaches a waveform synthesizer inverter comprising a series of cascaded programmable bilateral switch stages. When the switch stages are properly controlled, the inverter produces an output voltage having a stepped quasi-sinusoid waveform. The amplitude and frequency of the output waveform are varied by altering the timing and conduction duration of the switch stages. Although this configuration also provides control over the harmonic content of the output waveform, the configuration also requires several switch stages to limit harmonic distortion to acceptable levels.
Bowles (U.S. Pat. No. 5,757,633) teaches a multi-step inverter which employs multiple series-connected inverter bridges to piecewise approximate a sinusoidal output waveform. Each inverter bridge is bypassed or switched into service as required to create a portion of a stair-stepped sinusoidal waveform. Each inverter bridge is pulse-width modulated to smooth each step and thereby produce a smoother sinusoid. Although this configuration reduces switching losses, the configuration also requires several inverter bridges to limit harmonic distortion to acceptable levels.
Wobben (U.S. Pat. No. 6,452,819) teaches an inverter which uses asymmetrical harmonics generated at the output of the inverter stage to reduce harmonic distortion. At the inverter output, the inverter includes a three-phase output choke having a fourth choke leg. The asymmetrical magnetic fluxes produced in the fourth leg by the asymmetrical harmonics are collected by way of three resonant circuits, and then fed back to the negative bar of the DC voltage intermediate circuit which powers the inverter stage. The patentee discloses that the magnetic fluxes in the fourth choke leg flow back into the three main legs of the output choke, thereby increasing the output inductance of the choke. However, the specialized output choke increases the cost of the inverter.
Therefore, there remains a need for a mechanism for efficiently converting DC power into AC power without increasing harmonic distortion.
According to the present invention, there is provided an inverter which is configured to produce an output signal having a piece-wise linear sinusoidal, trapezoidal or clipped triangular waveform.
The inverter, according to one aspect of the present invention, includes a switch stage having a switch output, a switch controller coupled to the switch stage, and a filter coupled to the switch output. The switch stage includes switch means coupled to the switch output for switching the switch output between a pair of power supply rails. The switch controller is configured to cyclically linearly vary the duty cycle of the output signal at the switch output. The filter is configured to produce a piece-wise linear approximated sinusoidal output waveform from the output signal.
The inverter, according to another aspect of the present invention, includes a switch stage having a switch output, a switch controller coupled to the switch stage, and a filter coupled to the switch output. The switch stage includes switch means coupled to the switch output and configured to provide a switched constant-peak-amplitude output signal at the switch output. The switch controller is configured to cyclically linearly vary the duty cycle of the output signal. The filter is configured to produce a piece-wise linear approximated sinusoidal output waveform from the output signal.
According to the present invention, there is also provided a method for converting DC power into AC power by producing from a DC signal an AC signal having a piece-wise linear sinusoidal, trapezoidal or clipped triangular waveform.
The method involves the steps of (1) with switch means generating with a pulse-width modulated output signal, the output signal having a cyclically linearly varying duty cycle and an amplitude varying between a pair of signal levels; and (2) filtering the output signal in a manner to produce a piece-wise linear approximated sinusoidal output waveform.
In accordance with one embodiment of the inverter, the switch means includes a first electronic switch coupled to one of the power supply rails, and a second electronic switch coupled to the other of the power supply rails, and the switch output is coupled to the interconnection of the electronic switches. The switch controller is configured to vary the duty cycle in a manner such that the output waveform has a first plateau corresponding to a first power supply signal level, a second plateau corresponding to a second power supply signal level, and an intermediate portion varying linearly between the first plateau and the second plateau. During the first plateau, the duty cycle is 0% (the first electronic switch is fully on and the second electronic switch is fully off); whereas during the second plateau, the duty cycle is 100% (the first electronic switch is fully off and the second electronic switch is fully on).